1. Field of the Invention
This invention relates broadly to solid-state physics and to processes for producing ionic crystals having metal particles embedded therein. More particularly, it relates to the production of selected crystalline metallic oxides whose lattices contain dispersed precipitates of a selected metal.
2. Related Art
It is known that if colloidal precipitates having diameters in the range of about 1-200 nm are formed in ionic crystals, significant changes may take place in the physical properties of the crystals. For example, the presence of such precipitates may render the crystals useful for data-storage application or for the selective absorption of solar energy. The effects of metal colloids on ionic crystals, especially halides, are discussed in the following reference: A. E. Hughes and S. C. Jain, Adv. in Phys., 28, No. 6, pp. 717-828 (1979). Colloids of potassium, sodium and lithium have been formed in near-surface regions of MgO by ion implantation, followed by heat treatment. [M. Treilleux et al, J. Phys. Lett., Paris, 40 L-161 (1979)]. That method of forming the colloids is subject to the disadvantage that colloids are formed in only a very thin surface layer of the MgO; furthermore, subsequent annealing of the layer is required to minimize the damaging effects of the implantation.
Solar-selective films commonly are in the form of composite films which strongly absorb radiation over much or all of the solar spectrum but which are essentially transparent to longer-wavelength radiation. An ideal solar absorber has high absorption for photon wavelengths (.lambda.) less than 2.0 .mu.m (which band comprises most of the solar spectrum), and low absorption (or emissivity) for wavelengths exceeding 2.0 .mu.m, so that the absorber retains the absorbed energy. Such films, referred to as cermet films, can be deposited on a low-emissivity surface, such as a metallic mirror, by electroplating or by vapor deposition. The resulting article provides a combination whose absorbance spectrum approaches the form desired for a solar-selective surface. However, cermet films are subject to one or more of the following disadvantages: they are costly to produce, their composition cannot be varied easily to provide certain desired final properties, and they do not have the stability desired for long-term operation at high temperatures. For example, electroplated chrome is a commonly used selective surface, but it does not appear to be stable at temperatures above about 300.degree. C. Vapor-deposited composite metal insulator films are discussed in the following reference: H. G. Craigshead, R. Bartynski, R. A. Buhrman, L. Wojaik, and A. J. Sievers, Solar Energy Materials 1, pp. 105-124 (1979). That reference reports the production of Ni/Al.sub.2 O.sub.3 and Pt/Al.sub.2 O.sub.3 composite films by controlled co-evaporation onto fused quartz or polished Cu. The Ni/Al.sub.2 O.sub.3 composites comprised crystalline Ni particles in an amorphous matrix. As deposited on Cu substrates, both types of film exhibited an absorptivity (.alpha.) of .gtoreq.0.94 and an absorptivity to thermal-emissivity ratio (.alpha./E) of .gtoreq.13 at 150.degree. C. for an extended period, whereas a deposit of Pt/Al.sub.2 O.sub.3 remained stable at 600.degree. C.
The growth of MgO:Ni crystals by an arc-fusion technique and the characteristics of such crystals are described in the following references: J. Narayan and Y. Chen, J. App. Phys., 51(2), 1242-43 (1980); U.S. Pat. No. 3,829,391, "Submerged-Arc Process for Growing Transparent Alkaline-Earth Oxide Single Crystals," to Yok Chen and Marvin Abraham, issued Sept. 19, 1972.
Copending, co-assigned U.S. patent application Ser. No. 973,660, filed on Dec. 27, 1978, describes a process for producing alkaline-earth-oxide semiconductors. The process comprises contacting a metal-doped crystalline alkaline-earth-oxide with an oxygen-containing atmosphere at elevated temperature to form [Li].degree. defects therein and then quenching the heated material to render the defects stable at room temperature. For comparison, the application cites experiments in which Li-doped MgO was contacted with reducing atmospheres at elevated temperatures and then quenched. The resulting crystals were free from [Li].degree. defects, virtually colorless, and transparent.